Camera domes equipped with stepper motors are often used in security and surveillance systems. Often, these dome cameras are placed in outdoor locations to monitor parking lots, road traffic conditions, bridges and tunnels, and other public spaces.
However, in these outdoor locations, the dome cameras are frequently subjected to vibration caused by passing vehicles and wind. The vibrations unfortunately can result in positional changes of the stepper motor. Hereinafter, the term vibration, as it applies to the movement of the dome camera, refers to unwanted movement of a stepper motor from one positional step to another caused by external forces such as passing traffic and wind.
A stepper motor provides the motive force for panning the camera to capture an extended field of view. Additionally, the stepper motor is required to hold the camera in a fixed position. Holding the camera stationary is accomplished by adjusting the torque produced by the stepper motor to balance any external forces impacting the camera. Since the dome camera system may not have any position feedback system for determining the exact position of the camera, external vibrations or forces that are of significant strength can cause the stepper motor to jump to the next step in its rotation. This jump would result in the loss of positional accuracy. To prevent this, the holding current of the stepper motor is increased so that the holding torque is larger than the external forces or vibrations.
Maintaining the stepper motor continuously powered so that a holding torque is produced often can result in excessive heat being generated. Constant excess heat can have an adverse effect on the camera dome's operation as well as degrade the life of the stepper motor. Therefore, the stepper motor should ideally produce a holding torque with a minimum current, thus reducing the amount of heat produced.
However, as mentioned above, camera domes are subject to random vibrational forces induced by traffic, weather and other environmental conditions. A stepper motor that is providing only a minimal holding torque, in many cases, will not be able to counter these external forces, resulting in excessive camera dome motion and loss of positional accuracy. Hereinafter, the term vibration, as it applies to the movement of the dome camera, refers to unwanted movement of a stepper motor from one positional step to another caused by external forces such as passing traffic and wind.
Accordingly, a need exists for an apparatus and method for determining the minimum holding torque required to hold a dome camera stationary at any given moment.